This invention relates to a method of and apparatus for electron beam welding which is carried out by electron beams passing through magnetic shielding means provided for an electron gun and more particularly to a method of and apparatus for electron beam welding which can be effected continuously without interruption when dissimilar metals are welded.
In general, an electron beam welding apparatus is provided with an electron gun and a driving table disposed in vacuum. The electron gun emits electron beams to an abutment between two metals, whereby the metals are melted and welded at the abutment. In application of the electron beam welding to dissimilar metals, there arise shortcomings in that weld beads formed are deflected and welds formed tend to include nonwelded spots for the following reasons. Upon welding of the dissimilar metals, there will be created a hot junction and a cold junction within the materials being welded and thus the resulting thermal electromotive force will bring about the flow of electron currents. The electric current thus produced then brings about a stray magnetic field, such that weld beams are deflected midway due to the interaction between the component of a magnetic field that is in the direction of a welding axis and electron beams. In case where the metals are thin in thickness, even if the electron beams are deflected so that they will be emitted to a portion deviated from the abutment by a little, there is only a little problem because the heat due to emission of the electron beams is transferred to the abutment. In case where the metals are thick in thickness, the above-mentioned serious shortcomings take place.
Hitherto, in order to prevent the electron beams from deflecting when the dissimilar metals are welded, the electron beam welding apparatus is provided at the end of the electron gun with a magnetic shielding pipe made of a high-permeability magnetic material to prevent magnetic fields outside the pipe from reaching the electron beams inside it, and welding is carried out by straight electron beams passing through the magnetic shielding pipe. This kind of the electron beam welding apparatus is shown in FIG. 1 of Japanese Laying-open of Patent Application No. 50-129445 (1975). In this apparatus, in order to prevent the stray magnetic fields from reaching the electron beam inside the magnetic shielding pipe, it is necessary that a distance between the end of the magnetic shielding pipe and the dissimilar metals to be welded is not large, for example within about 10 mm.
On the other hand, spatter and metal vapor expelled from the weld formed around the abutment of the dissimilar metals by electron beam welding deposits on an end surface of the magnetic shielding pipe. The deposition is gradually accumulated and finally contacts with the weld swelled from the surface of the metals to a height of about 4 mm so that further continuation of the welding can not be effected. Therefore, it is necessary to clean up the end of the magnetic shielding pipe on which the deposition is accumulated, with interruption being taken for 2 to 3 minutes in welding of thin members and for 4 to 5 minutes in welding of thick members. The interruption of welding makes welding quality poor because crater takes place in the welding portion. Additionally, the interruption makes welding operation time longer. Therefore, it is desired that welding of the materials to be welded is completed without interruption.